This invention relates to an electromagnetic control device comprising a coil, a pole ring having annular pole faces, and an armature which is attracted to said pole faces on energisation of said coil. In particular it is concerned with a device of this type which has no air gap, that is to say one in which, when the coil is energised and the armature is attracted, there is no effective air gap between the armature and the pole ring. Typical structures which may embody gapless electromagnetic control devices of this kind are electromagnetic clutches and electromagnetic brakes.
In a device of the kind set forth the two ferromagnetic parts attracted together bear against one another in the energised condition with at least temporary frictional contact. A structure of this kind has of course the advantage that in the absence of the air gap comparatively high tractive forces can be applied, which promotes the transmission of a high degree of torque.
As a rule a frictional lining is arranged between the inner pole face or faces and the outer pole face or faces of the pole ring and this, in addition to providing magnetic insulation between the two poles, has the function of providing a maximum amount of friction. In particular the friction lining is intended to keep the wear of the pole faces within tolerable limits. The friction face of the friction lining is generally uniplanar with the faces of the outer pole and the inner pole.
In the case of gapless electromagnetic clutches or brakes of the kind referred to, however, high torque can only be transmitted when the magnetically effective pole faces themselves are large, this affording optimum magnetic conditions.
It has been found that the clutches or brakes of this type initially promote the transmission of a high degree of torque but this tends to tail off quickly. Wear is found to be very marked and scoring and cold welding occur between the coacting faces. Large dimension pole faces which are so effective magnetically are therefore not in practice successful. On the other hand the pole faces cannot be made too small, the resultant magnetic forces being proportional to the area of the pole faces. As a rule a compromise is sought to give an adequate length of life but with reduced torque transmission.
The object of this invention is to avoid the need for a compromise of this kind, which cannot always be successful.
A further object is to provide an electromagnetic clutch or brake of the kind set forth which has an optimum life combined with an optimum capability of torque transmission.